Interconnector material, intercellular separation structure, and solid electrolyte fuel cell

ABSTRACT

Provided is an interconnector material which is chemically stable in both oxidation atmospheres and reduction atmospheres, has a high electron conductivity (electric conductivity), a low ionic conductivity, does not contain Cr, and enables a reduction in sintering temperature. The interconnector material is arranged between a plurality of cells each composed of an anode layer, a solid electrolyte layer, and a cathode layer stacked sequentially, and electrically connects the plurality of cells to each other in series in a solid electrolyte fuel cell. The interconnector is formed of a ceramic composition represented by the composition formula La(Fe 1-x Al x )O 3  in which 0&lt;x&lt;0.5.

This is a division of application Ser. No. 13/005,649, filed Jan. 13,2011, which was continuation of application Serial No. PCT/JP2009/002566, filed Jun. 8, 2009, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an interconnector material, anintercellular separation structure formed by using the interconnectormaterial, and a solid electrolyte fuel cell including the intercellularseparation structure.

BACKGROUND ART

Generally, a planar solid electrolyte fuel cell (also referred to as asolid oxide fuel cell (SOFC)) is composed of a plurality of planarcells, as a power generating element, each composed of an anode (anegative electrode, a fuel electrode), a solid electrolyte and a cathode(a positive electrode, an air electrode) and interconnectors (alsoreferred to as separators). The interconnectors are arranged between theplurality of cells in order to electrically connect the plurality ofcells to each other in series and separate gases supplied to each of theplurality of cells; specifically, separate a fuel gas (e.g., hydrogen)as an anode gas supplied to an anode from an oxidant gas (e.g., air) asa cathode gas supplied to a cathode.

The interconnector needs to be chemically stable in a high-temperatureenvironment of 800 to 1000° C. which is an operating temperature of thesolid electrolyte fuel cell and in both oxidation and reductionatmospheres. Further, the interconnector material is desirably amaterial which has a high electric conductivity and can reduce an ohmicloss (IR loss).

In response to such requirements, the interconnector is conventionallyformed of heat resistant metal materials or conductive ceramic materialssuch as lanthanum chromite (LaCrO₃). When the interconnector is formedby use of such a conductive material, it is possible to make a memberfulfilling the above-mentioned functions of electrical connection andseparation of gases from one material. Generally, there have been useddense bodies of ceramic such as lanthanum chromite having a perovskitestructure doped with Sr, Ca or Mg as interconnector materials.

However, when sintering lanthanum chromite in the air in the methodsconventionally adopted in order to form an interconnector by usinglanthanum chromite, chromium oxide is evaporated from the lanthanumchromite powder or compounds containing volatile hexavalent chromium aresintered in the process of vaporization/recondensation. Therefore,densification resulting from diffusion within a particle is inhibitedand a gas-tight sintered body cannot be obtained.

In order to solve such problems, for example, a compound containing, asa dominant component, a composition represented by the compositionformula La_(1-x)Ca_(x)Cr_(1-y)O₃ (values of x and y satisfy 0<x≤0.4,0<y≤0.05, and y≤x) is proposed as a raw material powder of lanthanumchromite for forming a separator in Japanese Unexamined PatentPublication No. 4-119924 (hereinafter, referred to as Patent Document1). Patent Document 1 indicates that in this raw material powder oflanthanum chromite, the amount of chromium evaporation can be reduced bymaking the chromium content insufficient and thereby the sinteringproperty can be improved, and it becomes possible to realize a separatorhaving an excellent gas-tight property, and to achieve chemicalstability in the oxidation/reduction atmospheres and high electronconductivity, which are required for a separator.

Further, a compound containing, as a dominant component, a compositionrepresented by the composition formula Sr_(1-x)La_(x)TiO₃ (the value ofx satisfies 0<x≤0.3) is proposed, for example, in Japanese UnexaminedPatent Publication No. 2001-52725 (hereinafter, referred to as PatentDocument 2) as an interconnector material not containing Cr.

Moreover, a composition represented by the composition formula(La_(1-x)Sr_(x))(Fe_(1-y)Ti_(y))O₃ (values of x and y satisfy 0≤x≤0.1and 0<y<0.5) is proposed, for example, in Japanese Unexamined PatentPublication No. 2006-185697 (hereinafter, referred to as Patent Document3) as an interconnector material which has a high sintering property,can be sintered at 1400° C. or lower, and does not contain Cr.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Unexamined Patent Publication No. 4-119924

Patent Document 2: Japanese Unexamined Patent Publication No. 2001-52725

Patent Document 3: Japanese Unexamined Patent Publication No.2006-185697

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Since lanthanum chromite has a high sintering temperature, it isdifficult to co-sinter integrally with other materials for a fuelelectrode, a solid electrolyte, and an air electrode when being used forforming an interconnector. Therefore, the production efficiency of asolid electrolyte fuel cell deteriorates and production costs increase.Particularly, since lanthanum chromite is highly reactive with lanthanummanganite ((La, Sr)MnO₃) which is a material used for the air electrodeand counter diffusion occurs between Cr and Mn, there is a problem thata decomposition reaction is accelerated. Further, Sr-based lanthanumchromite needs to be sintered at a high temperature of 1600° C. orhigher to be densified. At this level of temperature, high electrodecharacteristics cannot be achieved since pores existing in the airelectrode and the fuel electrode disappear or ionic diffusion becomesremarkable in the materials for the air electrode or the fuel electrode.

Ca-based lanthanum chromite proposed in Patent Document 1 can bedensified by sintering at a low temperature of about 1300° C., but thesintering is performed in a liquid phase and therefore ion diffusionoccurs and reactivity of lanthanum chromite increases, so thatco-sintering with other materials for a fuel electrode, a solidelectrolyte, or an air electrode, cannot be effected.

Further, lanthanum chromite has an environmental problem since, forexample, SrCrO₄ and CaCrO₄ are produced as compounds of hexavalentchromium when producing lanthanum chromite.

On the other hand, the (Sr, La)TiO₃-based ceramic compositions proposedin Patent Document 2 is not preferred as an interconnector materialsince its electric conductivity in air at 900° C. is as low as about0.001 S/cm.

Further, (La, Sr)(Fe, Ti)O₃-based ceramic compositions proposed inPatent Document 3 are not preferred as an interconnector material sincetheir resistivity at 1000° C. is high, that is, electric conductivitythereof is low.

Thus, it is an object of the present invention to provide aninterconnector material which is chemically stable in both oxidation andreduction atmospheres, has a high electron conductivity (electricconductivity) and a low ionic conductivity, does not contain Cr, andenables a reduction in this sintering temperature; an intercellularseparation structure formed by using the interconnector material; and asolid electrolyte fuel cell including the intercellular separationstructure.

Means for Solving the Problems

The interconnector material according to the present invention can bearranged between a plurality of cells each composed of an anode layer, asolid electrolyte layer, and a cathode layer stacked sequentially, andelectrically connect the plurality of cells to each other in series in asolid electrolyte fuel cell, wherein a dominant component of thematerial is a ceramic composition represented by the composition formulaLa(Fe_(1-x)Al_(x))O₃ in which x represents a molar value satisfying0<x<0.5.

Since the interconnector material of the present invention contains aceramic composition having the above-mentioned limited composition as adominant component, it is chemically stable in both oxidation andreduction atmospheres, has a very low ionic conductivity and a highelectron conductivity (electric conductivity), and can be sintered at atemperature up to about 1300 to 1400° C.

In the interconnector material of the present invention, it is preferredthat the dominant component of the material is a ceramic compositionrepresented by the composition formula La(Fe_(1-x)Al_(x))O₃ in which xrepresents a molar amount satisfying 0.1≤x≤0.3.

If the composition of the interconnector material is further limited asdescribed above, a material which is chemically stable in reductionatmospheres at a temperature as high as 1000° C. can be obtained, andthe electron conductivity (electric conductivity) can be furtherincreased.

The intercellular separation structure according to the presentinvention can also be an intercellular separation structure which isarranged between a plurality of cells each composed of an anode layer, asolid electrolyte layer, and a cathode layer stacked sequentially in asolid electrolyte fuel cell, wherein the intercellular separationstructure includes an electrical insulator to separate an anode gas anda cathode gas which are supplied to each of the plurality of cells andan electrical conductor which is formed in the electrical insulator andelectrically connects the plurality of cells to each other, theelectrical insulator and the electrical conductor being formed byco-sintering, and wherein the electrical conductor is preferably formedof the interconnector material having the above-mentionedcharacteristics.

By employing such a constitution, an intercellular separation structure,which is chemically stable in a high-temperature environment of 800 to1000° C., an operating temperature of the solid electrolyte fuel cell,and in both oxidation and reduction atmospheres, can be obtained byco-sintering at a low temperature of about 1300 to 1400° C.

In addition, the electrical conductor in the intercellular separationstructure of the present invention may be partially formed of theinterconnector material having the above-mentioned characteristics. Inthis case, the part of the electrical conductor of the interconnectormaterial may be formed on the anode layer side or the cathode layer sideto contact with an anode gas or a cathode gas, or may be formed at anintermediate portion of the electrical conductor.

By employing such a constitution, the size of the dense portion, throughwhich a gas does not pass and which is formed of the interconnectormaterial having the above-mentioned characteristics, is reduced, andthereby thermal stress produced during fabricating (co-sintering) theintercellular separation structure or during operating a solidelectrolyte fuel cell can be mitigated. Further, in the above-mentionedelectrical conductor, a material having a smaller electrical resistancethan that of the interconnector material having the above-mentionedcharacteristics can be selected and used as a material for an electronflow path.

The solid electrolyte fuel cell according to one aspect of the presentinvention includes a plurality of cells each composed of an anode layer,a solid electrolyte layer and a cathode layer stacked sequentially, andthe intercellular separation structure having the above-mentionedcharacteristics arranged between the plurality of cells.

By employing such a constitution, it is possible to obtain a solidelectrolyte fuel cell including an intercellular separation structure,which is chemically stable in a high-temperature environment of 800 to1000° C., an operating temperature of the solid electrolyte fuel cell,and in both oxidation and reduction atmospheres, and it is possible toobtain a solid electrolyte fuel cell including an interconnector whichis particularly good in joining properties with the air electrode sincean insulating layer is not formed at a junction interface between theinterconnector and the air electrode.

The solid electrolyte fuel cell according to another aspect of thepresent invention may include a plurality of cells each composed of ananode layer, a solid electrolyte layer and a cathode layer stackedsequentially, and an intercellular separation structure which isarranged between the plurality of cells and at least a conductivematerial layer formed of the interconnector material containing aceramic composition represented by the composition formulaLa(Fe_(1-x)Al_(x))O₃ in which x represents a molar amount ratiosatisfying 0<x<0.5 as a dominant component.

By employing such a constitution, it is possible to obtain a solidelectrolyte fuel cell including an intercellular separation structureincluding a conductive material layer which is chemically stable in bothoxidation and reduction atmospheres, has a very low ionic conductivityand a high electron conductivity (electric conductivity), and can reducethe sintering temperature to about 1300 to 1400° C.

In the solid electrolyte fuel cell according to another aspect of thepresent invention, it is preferred that the anode layer contains nickel,an intermediate layer is formed between the conductive material layerand the anode layer, and the intermediate layer is composed of atitanium-based perovskite oxide containing at least one element selectedfrom the group consisting of strontium, calcium and barium.

By employing such a constitution, it is possible to prevent Fe containedin the interconnector material from reacting with Ni contained in theanode layer to produce a high-resistive phase at a junction interfacebetween the conductive material layer and the anode layer when forming,by co-sintering, the anode layer containing nickel (Ni) and theconductive material layer formed of the interconnector materialcontaining a ceramic composition represented by the composition formulaLa(Fe_(1-x)Al_(x))O₃ in which x is 0<x<0.5 as a dominant component.Thereby, it is possible to improve the electrical connection of theanode layer containing nickel to the conductive material layer in theintercellular separation structure.

Furthermore, according to another aspect of the present invention, it ispossible to form the plurality of cells and the intercellular separationstructure by co-sintering, by employing such a constitution.

Furthermore, according to still another aspect of the present invention,it is preferred that the intermediate layer has pores.

In this case, since there is no need to densify the intermediate layer,and production of a cell structure is easy.

In the solid electrolyte fuel cell according to another aspect of thepresent invention, it is preferred that the titanium-based perovskiteoxide is a perovskite oxide represented by A_(1-x)B_(x)Ti_(1-y)C_(y)O₃in which A represents at least one selected from the group consisting ofSr, Ca and Ba, B represents a rare-earth element, C represents Nb or Ta,and x and y each represent mols, satisfying 0≤x≤0.5 and 0≤y≤0.5.

In this case, B is preferably La or Y. By substituting La or Y of therare-earth elements for part of A, the electric conductivity of theintermediate layer can be increased.

Effects of the Invention

According to the present invention, it is possible to obtain aninterconnector material which is chemically stable in both oxidation andreduction atmospheres, has a low ionic conductivity and a high electronconductivity (electric conductivity), and can be densified at a lowtemperature of about 1300 to 1400° C. Further, by using theinterconnector material, it is possible to obtain an intercellularseparation structure, which is chemically stable in the high-temperatureoperating environment of 800 to 1000° C. being an of the solidelectrolyte fuel cell and in both oxidation and reduction atmospheres,and a solid electrolyte fuel cell including the intercellular separationstructure.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a graph showing one example of the peak intensity, obtained byXRD, of each phase of a bulk sample of an interconnector material ofsample No. 3 (x=0.1) prepared in an example.

FIG. 2 is a graph showing a relationship between temperature (° C.) andthermal expansion (ΔL/L) (%) in bulk samples of an interconnector, asolid electrolyte layer and a support structure.

FIG. 3 is an exploded perspective view showing, in a disassembled state,members constituting a planar solid electrolyte fuel cell as anembodiment of the present invention.

FIG. 4 is an exploded perspective view showing, in a disassembled state,a state in which sheets constituting a planar solid electrolyte fuelcell are stacked as an embodiment of the present invention.

FIG. 5 is a sectional view schematically showing a cross-section of aplanar solid electrolyte fuel cell as an embodiment of the presentinvention.

FIG. 6 is an exploded perspective view showing, in a disassembled state,members constituting a planar solid electrolyte fuel cell as anotherembodiment of the present invention and as a sample prepared in anexample of the present invention.

FIG. 7 is an exploded perspective view showing, in a disassembled state,a state in which sheets constituting a planar solid electrolyte fuelcell are stacked as another embodiment of the present invention and as asample prepared in an example of the present invention.

FIG. 8 is a sectional view schematically showing a cross-section of aplanar solid electrolyte fuel cell as another embodiment of the presentinvention and as a sample prepared in an example of the presentinvention.

FIG. 9 is a sectional view schematically showing a cross-section of aplanar solid electrolyte fuel cell as an example in which a part of theelectrical conductor is formed of the interconnector material of thepresent invention.

FIG. 10 is a sectional view schematically showing a cross-section of aplanar solid electrolyte fuel cell as another example in which a part ofthe electrical conductor is formed of the interconnector material of thepresent invention.

FIG. 11 is a sectional view schematically showing a cross-section of aplanar solid electrolyte fuel cell as another example in which a part ofthe electrical conductor is formed of the interconnector material of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present inventors made investigations from various standpoints inorder to attain, an interconnector material for a solid electrolyte fuelcell which is arranged between a plurality of cells each composed of ananode layer, a solid electrolyte layer and a cathode layer stackedsequentially, and electrically connects the plurality of cells to eachother in series in a solid electrolyte fuel cell, and which ischemically stable in both oxidation and reduction atmospheres, has ahigh electron conductivity (electric conductivity) and a low ionicconductivity, and has a composition not containing Cr, and which enablesa reduction in sintering temperature.

Based on these investigations, the present inventor considered using aceramic composition represented by the composition formulaLa(Fe_(1-x)Al_(x))O₃ as the interconnector material for the solidelectrolyte fuel cell.

Then, the present inventor prepared ceramic compositions represented bythe composition formula La(Fe_(1-x)Al_(x))O₃ at various compositionratios. Consequently, the present inventors found that when x satisfies0<x<0.5 in the ceramic composition represented by the compositionformula La(Fe_(1-x)Al_(x))O₃, in which x represents a molar amount, theinterconnector material is chemically stable in both oxidation andreduction atmospheres, has a low ionic conductivity and a high electronconductivity (electric conductivity), and can reduce the sinteringtemperature to about 1300 to 1400° C.

Further, it was found that when x satisfies 0.1≤x≤0.3 in the ceramiccomposition represented by the composition formula La(Fe_(1-x)Al_(x))O₃,an interconnector material which is chemically stable even inhigh-temperature reduction atmospheres of 1000° C. can be attained andthe electron conductivity (electric conductivity) of the material canalso be made higher.

When the interconnector material contains a ceramic composition havingthe above-mentioned limited composition as a dominant component, it ispossible to obtain an interconnector material which is chemically stablein both oxidation and reduction atmospheres, has a very low ionicconductivity and a high electron conductivity (electric conductivity),and can be densified at a low sintering temperature of about 1300 to1400° C. Since the ionic conductivity is very low, loss resulting from acounter electromotive force generated in connecting the cells to eachother can be neglected. Further, since the interconnector material ofthe present invention can be densified by sintering at a temperature of1400° C. or lower, pores in the air electrode can be retained.

Moreover, the above-mentioned interconnector materials can be used formaterials of the electrical conductor contained in an intercellularseparation structure which is arranged between a plurality of cells eachcomposed of an anode layer, a solid electrolyte layer, and a cathodelayer stacked sequentially in the solid electrolyte fuel cell as anembodiment of the present invention. The intercellular separationstructure can include an electrical insulator to separate an anode gasand a cathode gas which are supplied to each of the plurality of cellsand an electrical conductor which is formed in the electrical insulatorand electrically connects the plurality of cells to each other, and theelectrical insulator and the electrical conductor can be formed byco-sintering. When the electrical conductor is formed of theinterconnector material having the above-mentioned characteristics, anintercellular separation structure which is chemically stable in thehigh-temperature operating environment of 800 to 1000° C. temperature ofthe solid electrolyte fuel cell and in both oxidation and reductionatmospheres, can be obtained by co-sintering at a low temperature ofabout 1300 to 1400° C.

Moreover, the solid electrolyte fuel cell as an embodiment of thepresent invention includes a plurality of cells each composed of ananode layer, a solid electrolyte layer and a cathode layer stackedsequentially and an intercellular separation structure arranged betweenthe plurality of cells, and the plurality of cells and the intercellularseparation structure are formed by co-sintering. When the electricalconductor to constitute a part of the intercellular separation structureis formed of the interconnector material having the above-mentionedcharacteristics, it is possible to obtain a solid electrolyte fuel cellincluding an intercellular separation structure which is chemicallystable in a high-temperature environment of 800 to 1000° C. and in bothoxidation and reduction atmospheres. Particularly, since theinterconnector is composed of a material whose dominant component is aperovskite phase, the solid electrolyte fuel cell can include theinterconnector which is good in joining properties with the cathodelayer composed of a material containing La_(1-x)Sr_(x)MnO₃ and the like,and further, the solid electrolyte fuel cell, in which an insulatinglayer is not formed at a junction interface between the interconnectorand the cathode layer, can be obtained. As a result, there is noincrease in electrical resistance due to the reaction between theinterconnector and the air electrode. Since the interconnector iscomposed of a material not reacting with zirconia, the interconnectorcan be joined to each of the solid electrolyte layer, the cathode layerand the anode layer each containing zirconia without degrading thecharacteristics of these layers.

The solid electrolyte fuel cell as another embodiment of the presentinvention includes a plurality of cells each composed of an anode layer,a solid electrolyte layer and a cathode layer stacked sequentially, andan intercellular separation structure which is arranged between theplurality of cells and includes at least a conductive material layerformed of the interconnector material containing a ceramic compositionrepresented by the composition formula La(Fe_(1-x)Al_(x))O₃ in which xrepresents a molar amount satisfying 0<x<0.5 as a dominant component.

By employing such a constitution, it is possible to obtain a solidelectrolyte fuel cell including an intercellular separation structurehaving a conductive material layer which is chemically stable in bothoxidation and reduction atmospheres, has a very low ionic conductivityand a high electron conductivity (electric conductivity), and can reducethe sintering temperature to about 1300 to 1400° C.

In the solid electrolyte fuel cell as another embodiment of the presentinvention, it is preferred that the anode layer contains nickel, anintermediate layer is formed between the conductive material layer andthe anode layer, and the intermediate layer is composed of atitanium-based perovskite oxide containing at least one element selectedfrom the group consisting of strontium, calcium and barium.

By employing such a constitution, it is possible to prevent Fe containedin the interconnector material from reacting with Ni contained in theanode layer to produce a high-resistive phase at a junction interfacebetween the conductive material layer and the anode layer when forming,by co-sintering, the anode layer containing nickel (Ni) and theconductive material layer formed of the interconnector materialcontaining a ceramic composition represented by the composition formulaLa(Fe_(1-x)Al_(x))O₃ in which 0<x<0.5 as a dominant component. Thereby,it is possible to improve the electrical connection of the anode layercontaining nickel to the conductive material layer in the intercellularseparation structure.

Further, in another embodiment of the present invention, by employingthe above-mentioned constitution, it is possible to form the pluralityof cells and the intercellular separation structure by co-sintering.

In the solid electrolyte fuel cell as another embodiment of the presentinvention, the intermediate layer preferably has pores. In this case,since there is no need to densify the intermediate layer, production ofa cell structure is easy.

In another embodiment of the present invention, the titanium-basedperovskite oxide is preferably a perovskite oxide represented byA_(1-x)B_(x)Ti_(1-y)C_(y)O₃ in which A represents at least one selectedfrom the group consisting of Sr, Ca and Ba, B represents a rare-earthelement, C represents Nb or Ta, and x and y each represent a molaramount satisfying 0≤x≤0.5 and 0≤y≤0.5.

In this case, B is preferably La or Y. By substituting La or Y of therare-earth elements for part of A, the electric conductivity of theintermediate layer can be increased.

Hereinafter, the constitutions of the solid electrolyte fuel cell asembodiments of the present invention will be described with reference tothe drawings.

As shown in FIGS. 3 to 5, a solid electrolyte fuel cell 1 as anembodiment of the present invention includes a plurality of cells eachcomposed of a fuel electrode layer 11 as an anode layer, a solidelectrolyte layer 12 and an air electrode layer 13 as a cathode layer,and an intercellular separation structure arranged between the pluralityof cells. The intercellular separation structure includes a supportstructure 14 composed of an electrical insulator to separate a fuel gasas an anode gas and air as a cathode gas which are supplied to each ofthe plurality of cells, and an interconnector 15 as an electricalconductor which is formed in the support structure 14 and electricallyconnects the plurality of cells to each other. The interconnector 15 isformed by using a ceramic composition represented by the compositionformula La(Fe_(1-x)Al_(x))O₃. The solid electrolyte fuel cell 1 shown inFIG. 5 is a battery including a single cell and the intercellularseparation structures are disposed on both sides of the cell.Furthermore, a current collecting layer 31 of fuel electrode is disposedbetween the fuel electrode layer 11 and the interconnector 15, and acurrent collecting layer 32 of air electrode is disposed between the airelectrode layer 13 and the interconnector 15.

The solid electrolyte fuel cell 1 as an embodiment of the presentinvention is fabricated in the following manner.

First, as shown by dashed lines in FIG. 3, through-holes 15 a forfilling green sheets of the plurality of interconnectors 15 are formedin the green sheet of the support structure 14 constituting theintercellular separation structure.

Further, as shown by dashed lines in FIG. 3, elongated through-holes 21a and 22 a respectively for forming a fuel gas supply channel 21 and anair supply channel 22 shown in FIG. 4 are formed in the green sheet ofthe support structure 14 by puncturing the green sheet with a mechanicalpuncher.

Moreover, fitting portions 11 a, 12 a and 13 a into which the greensheets of the fuel electrode layer 11, the solid electrolyte layer 12and the air electrode layer 13 are to be fitted, respectively, areformed in the green sheet of the support structure 14 on which the fuelelectrode layer 11, the solid electrolyte layer 12 and the air electrodelayer 13 are to be arranged.

Moreover, fitting portions 31 a and 32 a into which the green sheets ofthe current collecting layer 31 of fuel electrode and the currentcollecting layer 32 of air electrode are to be fitted, respectively, areformed in the green sheet of the support structure 14 on which thecurrent collecting layer 31 of fuel electrode and the current collectinglayer 32 of air electrode are to be arranged. The green sheet of thecurrent collecting layer 31 of fuel electrode is prepared by using amaterial having the same composition as that of the material powder ofthe fuel electrode layer 11 and the green sheet of the currentcollecting layer 32 of air electrode is prepared by using a materialhaving the same composition as that of the material powder of the airelectrode layer 13.

In the green sheets of the support structures 14 thus prepared, thegreen sheet of the interconnector 15 is fitted into the through-hole 15a, the green sheets of the fuel electrode layer 11, the solidelectrolyte layer 12 and the air electrode layer 13 are fitted into thefitting portions 11 a, 12 a and 13 a, respectively, and the green sheetsof the current collecting layer 31 of fuel electrode and the currentcollecting layer 32 of air electrode are fitted into the fittingportions 31 a and 32 a, respectively. Five green sheets thus obtainedare stacked sequentially as shown in FIG. 4.

The stacked layers are press-bonded to one another at a predeterminedpressure by warm isostatic pressing (WIP) at a predetermined temperaturefor a predetermined time. The press-bonded body is degreased within apredetermined temperature range, and then is sintered by beingmaintained at a predetermined temperature for a predetermined time.

The solid electrolyte fuel cell 1 as an embodiment of the presentinvention is thus fabricated.

As shown in FIGS. 6 to 8, the solid electrolyte fuel cell 1 as anotherembodiment of the present invention includes a plurality of cells eachcomposed of a fuel electrode layer 11 as an anode layer, a solidelectrolyte layer 12 and an air electrode layer 13 as a cathode layer,and an intercellular separation structure arranged between the pluralityof cells. Herein, the fuel electrode layer 11 contains nickel. A supportstructure 14, which is composed of an electrical insulator to separate afuel gas as an anode gas and air as a cathode gas which are supplied toeach of the plurality of cells, is formed outside the plurality ofcells. The intercellular separation structure includes an interconnector15 as an electrical conductor which electrically connects the pluralityof cells to each other. The interconnector 15 is formed by using aceramic composition represented by the composition formulaLa(Fe_(1-x)Al_(x))O₃. The solid electrolyte fuel cell 1 shown in FIG. 8is a battery including a single cell and intercellular separationstructures are disposed on both sides of the cell. Furthermore, acurrent collecting layer 31 of fuel electrode is disposed between thefuel electrode layer 11 and the interconnector 15, and a currentcollecting layer 32 of air electrode is disposed between the airelectrode layer 13 and the interconnector 15. The current collectinglayer 31 of fuel electrode is prepared by using a material having thesame composition as that of the material of the fuel electrode layer 11and the current collecting layer 32 of air electrode is prepared byusing a material having the same composition as that of the material ofthe air electrode layer 13. An intermediate layer 18 is disposed betweenthe interconnector 15 and the fuel electrode layer 11, specifically,between the interconnector 15 and the current collecting layer 31 offuel electrode. The intermediate layer 18 is formed by using atitanium-based perovskite oxide represented byA_(1-x)B_(x)Ti_(1-y)C_(y)O₃ in which A represents at least one selectedfrom the group consisting of Sr, Ca and Ba, B represents a rare-earthelement, C represents Nb or Ta, 0≤x≤0.5 and 0≤y≤0.5, for example,SrTiO₃.

When thus subjecting the interconnector 15 composed of the ceramiccomposition represented by the composition formula La(Fe_(1-x)Al_(x))O₃,and the fuel electrode layer 11/the current collecting layer 31 of fuelelectrode, which contain nickel, to co-sintering, an intermediate layer18, which is composed of a titanium-based perovskite oxide representedby SrTiO₃, for example, is disposed between the interconnector 15 andthe fuel electrode layer 11/the current collecting layer 31 of fuelelectrode for the purpose of preventing Fe contained in theinterconnector 15 from reacting with Ni contained in the fuel electrodelayer 11 and the current collecting layer 31 of fuel electrode. Herein,the interconnector 15 is formed densely so as to have a high electricconductivity, in other words, a small electrical resistance, and preventthe permeation of air or a fuel gas. The material composing theintermediate layer 18 does not have to be dense, and may be porous.

It is based on the following findings of the present inventors that theintermediate layer 18 composed of a titanium-based perovskite oxide isdisposed in between the interconnector 15 composed of the ceramiccomposition represented by the composition formula La(Fe_(1-x)Al_(x))O₃and the fuel electrode layer 11/the current collecting layer 31 of fuelelectrode, which contain nickel, as described above.

When the interconnector 15, which is composed of the ceramic compositionrepresented by the composition formula La(Fe_(1-x)Al_(x))O₃, was joinedto the fuel electrode layer 11 containing nickel through co-sintering,Fe reacted with Ni to produce LaAlO₃ having a smaller Fe content at ajunction portion (interface). LaAlO₃ has a low electric conductivity andinterfered with electric joining between the interconnector 15 composedof the ceramic composition represented by the composition formulaLa(Fe_(1-x)Al_(x))O₃ and the fuel electrode layer 11 containing nickel.Thus, the intermediate layer 18, which is composed of a titanium-basedperovskite oxide in which an electric conductivity is increased in afuel atmosphere, for example, SrTiO₃, was disposed, and consequentlygood electrical connection was attained. This is because SrTiO₃, forexample, which is one of compounds represented byA_(1-x)B_(x)Ti_(1-y)C_(y)O₃, in which A represents at least one selectedfrom the group consisting of Sr, Ca and Ba, B represents a rare-earthelement, C represents Nb or Ta, 0≤x≤0.5 and 0≤y≤0.5, composing theintermediate layer 18, does not form a high-resistive layer even if itis subjected to co-sintering together with the interconnector 15composed of the ceramic composition represented by the compositionformula La(Fe_(1-x)Al_(x))O₃ and the fuel electrode layer 11 containingnickel.

The solid electrolyte fuel cell 1 as another embodiment of the presentinvention is fabricated in the following manner.

First, as shown by dashed lines in FIG. 6, elongated through-holes 21 aand 22 a respectively for forming a fuel gas supply channel 21 and anair supply channel 22 shown in FIG. 7 are formed in the green sheet ofthe support structure 14 by puncturing the green sheet with a mechanicalpuncher.

Further, fitting portions 11 a, 12 a and 13 a into which the greensheets of the fuel electrode layer 11, the solid electrolyte layer 12and the air electrode layer 13 are to be fitted, respectively, areformed in the green sheet of the support structure 14 on which the fuelelectrode layer 11, the solid electrolyte layer 12 and the air electrodelayer 13 are to be arranged.

Moreover, fitting portions 31 a and 32 a into which the green sheets ofthe current collecting layer 31 of fuel electrode and the currentcollecting layer 32 of air electrode are to be fitted, respectively, areformed in the green sheet of the support structure 14 on which thecurrent collecting layer 31 of fuel electrode and the current collectinglayer 32 of air electrode are to be arranged. The green sheet of thecurrent collecting layer 31 of fuel electrode is prepared by using amaterial having the same composition as that of the material powder ofthe fuel electrode layer 11 and the green sheet of the currentcollecting layer 32 of air electrode is prepared by using a materialhaving the same composition as that of the material powder of the airelectrode layer 13.

Furthermore, as shown by dashed lines in FIG. 6, elongated through-holes21 a and 22 a respectively for forming a fuel gas supply channel 21 andan air supply channel 22 shown in FIG. 7 are formed in the green sheetsof the interconnector 15 and the intermediate layer 18 by puncturing thegreen sheets with a mechanical puncher.

In the green sheets of the support structures 14 thus prepared, thegreen sheets of the fuel electrode layer 11, the solid electrolyte layer12 and the air electrode layer 13 are fitted into the fitting portions11 a, 12 a and 13 a, respectively, and the green sheets of the currentcollecting layer 31 of fuel electrode and the current collecting layer32 of air electrode are fitted into the fitting portions 31 a and 32 a,respectively. The green sheets of the interconnector 15 and theintermediate layer 18 are stacked sequentially on three green sheetsthus obtained as shown in FIG. 7.

The stacked layers are press-bonded to one another at a predeterminedpressure by warm isostatic pressing (WIP) at a predetermined temperaturefor a predetermined time. The press-bonded body is degreased within apredetermined temperature range, and then is sintered by beingmaintained at a predetermined temperature for a predetermined time.

The solid electrolyte fuel cell 1 as another embodiment of the presentinvention is thus fabricated.

In the above-mentioned embodiment, the entire electrical conductor whichelectrically connects the plurality of cells to each other is composedof the interconnector 15 formed of the interconnector material of thepresent invention as shown in FIGS. 5 and 8, but only a part of theelectrical conductor may be formed of the interconnector material of thepresent invention.

FIGS. 9 to 11 are sectional views each schematically showing across-section of a planar solid electrolyte fuel cell as some examplesin which a part of the electrical conductor is formed of theinterconnector material of the present invention.

As shown in FIG. 9, the intercellular separation structure includes asupport structure 14 composed of an electrical insulator to separate afuel gas as an anode gas and air as a cathode gas which are supplied toeach of the plurality of cells, an interconnector 15, which is composedof the interconnector material of the present invention, as anelectrical conductor which is formed in the support structure 14 andelectrically connects the plurality of cells to each other, and aconductive body 16 for an interconnector formed so as to connect to theinterconnector 15. The interconnector 15 is formed on the air electrodelayer 13 side so as to contact with air. More specifically, it is formedso as to connect to the air electrode layer 13 through the currentcollecting layer 32 of air electrode. The conductive body 16 for aninterconnector is formed so as to contact with a fuel gas. Morespecifically, it is formed so as to connect to the fuel electrode layer11 through the current collecting layer 31 of fuel electrode, and ismade of, for example, a mixture of nickel oxide (NiO) andyttria-stabilized zirconia (YSZ).

Further, as shown in FIG. 10, the intercellular separation structureincludes a support structure 14 composed of an electrical insulator toseparate a fuel gas as an anode gas and air as a cathode gas which aresupplied to each of the plurality of cells, an interconnector 15, whichis composed of the interconnector material of the present invention, asan electrical conductor which is formed in the support structure 14 andelectrically connects the plurality of cells to each other, and aconductive body 17 for an interconnector formed so as to connect to theinterconnector 15. The interconnector 15 is formed on the fuel electrodelayer 11 side so as to contact with a fuel gas. More specifically, it isformed so as to connect to the fuel electrode layer 11 through thecurrent collecting layer 31 of fuel electrode. The conductive body 17for an interconnector is formed so as to contact with air. Morespecifically, it is formed so as to connect to the air electrode layer13 through the current collecting layer 32 of air electrode, and is madeof, for example, a mixture of lanthanum manganite ((La, Sr)MnO₃) andyttria-stabilized zirconia (YSZ).

Furthermore, as shown in FIG. 11, an intercellular separation structureincludes a support structure 14 composed of an electrical insulator toseparate a fuel gas as an anode gas and air as a cathode gas which aresupplied to each of the plurality of cells, an interconnector 15, whichis composed of the interconnector material of the present invention, asan electrical conductor which is formed in the support structure 14 andelectrically connects the plurality of cells to each other, andconductive bodies 16 and 17 for an interconnector formed so as toconnect to the interconnector 15. The conductive body 16 for aninterconnector is formed so as to contact with a fuel gas. Morespecifically, it is formed so as to connect to the fuel electrode layer11 through the current collecting layer 31 of fuel electrode, and ismade of, for example, a mixture of nickel oxide (NiO) andyttria-stabilized zirconia (YSZ). The conductive body 17 for aninterconnector is formed so as to contact with air. More specifically,it is formed so as to connect to the air electrode layer 13 through thecurrent collecting layer 32 of air electrode, and is made of, forexample, a mixture of lanthanum manganite ((La, Sr)MnO₃) andyttria-stabilized zirconia (YSZ). The interconnector 15 is formed so asto connect the conductive bodies 16 and 17 for an interconnector to eachother.

As shown in FIGS. 9 to 11, the interconnector 15 in the intercellularseparation structure of the present invention, which is formed of theinterconnector material of the present invention, may be formed on thefuel electrode layer 11 (as an anode layer) side or the air electrodelayer 13 (as a cathode layer) side, as shown in FIG. 9 or 10, to contactwith a fuel gas as an anode gas or air as a cathode gas, or may beformed at an intermediate portion of the electrical conductor, as shownin FIG. 11.

By employing such a constitution, the size of a dense portion throughwhich a gas does not pass and which is formed of the interconnectormaterial of the present invention is reduced, and thereby thermal stressproduced during fabricating (co-sintering) the intercellular separationstructure or during operating the solid electrolyte fuel cell can bemitigated. Further, a material having a smaller electrical resistancethan that of the interconnector material of the present invention can beselected and used as a material composing an electron flow path in theabove-mentioned electrical conductor.

For example, the green sheet of an intercellular separation structureshown in FIG. 9 is fabricated in the following manner. First, the greensheet for a support structure 14 is prepared. Through-holes are formedin the green sheet for the support structure 14, and a mixed paste ofnickel oxide (NiO) and zirconia stabilized with 8 mol % of yttria (YSZ)is filled into the through-holes. This paste is prepared by mixing NiO,YSZ and a vehicle in proportions of 80 parts by weight:20 parts byweight:60 parts by weight and kneading the mixture with a three-rollkneader. As the vehicle, a mixture of ethyl cellulose and a solvent,trade name EC-200 FTR manufactured by Nisshin Kasei Co., Ltd., is used.In the meantime, a green sheet for the interconnector 15 is prepared.Then, the green sheet for the interconnector 15 is cut into thedisc-shape shown in FIG. 3 so as to have a larger diameter than that ofthe through-hole, and the disc-shaped green sheet for the interconnector15 is press-bonded to an air electrode side of a through-hole portion ofa green sheet for the support structure 14. Further, in order to preparethe green sheet of the intercellular separation structure shown in FIG.8, two green sheets for the support structure 14 are prepared and thedisc-shaped green sheet for the interconnector 15 is press-bonded to thegreen sheets for the support structure 14 with the disc-shaped greensheet for the interconnector 15 interposed between the two green sheetsfor the support structure 14.

In the above-mentioned embodiment, an example in which theinterconnector material of the present invention is applied to aninterconnector of a planar solid electrolyte fuel cell and anintercellular separation structure including the interconnector has beendescribed, but the interconnector material of the present invention canalso be applied to an interconnector formed at a part on a cylindricalouter circumferential face in a cylindrical solid electrolyte fuel cell,an interconnector formed on a flat surface in a flat tube-shaped solidelectrolyte fuel cell, and interconnectors having various shapes otherthan the above-mentioned shapes.

EXAMPLES

Hereinafter, examples of the present invention will be described.

First, bulk samples of ceramic compositions represented by thecomposition formula La(Fe_(1-x)Al_(x))O₃ were prepared at variouscomposition ratios as interconnector materials according to thefollowing procedure, and each sample was evaluated.

(Preparation of Bulk Sample)

As starting materials of the samples of sample Nos. 1 to 9, lanthanumoxide (La₂O₃), iron oxide (Fe₂O₃), and aluminum oxide (Al₂O₃) wereweighed stoichiometrically and mixed so that the values of x, which is amolar amount in the composition formula La(Fe_(1-x)Al_(x))O₃, are valuesshown in Table 1. Water was added to the mixtures, and the resultingmixtures were milled with zirconia balls and mixed. Thereafter, theresulting mixed powders were dried and calcined at 1100° C. An organicsolvent and a butyral-based binder were added to the obtained calcinedpowders, and the resulting mixtures were mixed to prepare slurries. Theslurries were formed into sheets by a the doctor blade method. Theresulting green sheets were subjected to binder removal treatment, andthen were sintered by being maintained at 1300° C. and at 1400° C. toobtain samples. The following evaluations were performed with theobtained samples.

In Table 1, x in each of the composition formulas of the samples ofsample Nos. 2 to 7 satisfies 0.05≤x≤0.4 (within the scope of the presentinvention), and x in each of the composition formulas of the samples ofsample Nos. 1, 8 and 9 is 0, 0.5, or 1 (out of the scope of the presentinvention). The samples of sample Nos. 2 to 8 were evaluated on thefollowing properties (1) to (6) and the samples of sample Nos. 1 and 9were evaluated on the following properties (1) to (5).

(Evaluation of Bulk Sample of Material for Interconnector) (1) X-RayDiffraction

The produced phase was identified by performing a powder X-raydiffraction analysis (XRD, CuKα rays) after calcination and aftersintering of the sample. It was identified that all of the phasesproduced after sintering were a single phase of a perovskite structure.

(2) Sintering Property

Densities of the sintered samples were measured according to theArchimedes method. The sintering properties of the samples wereevaluated depending on whether relative densities measured aftersintering the samples at 1300° C. and at 1400° C. are 92% or more, ornot. In Table 1, the case where the relative density measured aftersintering the samples at 1300° C. or 1400° C. is 92% or more isrepresented by “good” in the box of “Sintering property (1300° C.)” or“Sintering property (1400° C.)”, and the case where the relative densityis less than 92% is represented by “poor”.

(3) Electric Conductivity

With respect to the sintered samples, the electric conductivities in anoxidation atmosphere (900° C. in the atmospheric air) and in a reductionatmosphere (hydrogen gas containing about 4% of water vapor), and theelectric conductivities in those atmospheres at 1000° C. were measuredby an alternating current four probe analysis. The electric conductivitydecreased as x increased. When power is generated at a current densityof 0.3 A/cm² by a planar solid electrolyte fuel cell and the thicknessof the sample is 40 μm, an electric conductivity of 0.025 Scm⁻¹ or moreis required so as to achieve an ohmic loss of 50 mV or less. When theinterconnector is interposed between insulative support structures inthe fuel cell, an electric conductivity of 0.05 Scm⁻¹ or more, which istwo times as large as the above-mentioned electric conductivity, isrequired if the proportion of the interconnector is 50% by volume orless. In consideration of the above, the sample in which the electricconductivity is 0.05 Scm⁻¹ or more is represented by “very good”, thesample in which the electric conductivity is 0.025 Scm⁻¹ or more andless than 0.05 Scm⁻¹ is represented by “good”, and the sample in whichthe electric conductivity is less than 0.025 Scm⁻¹ is represented by“poor” in Table 1.

(4) Reduction Stability

Powder X-ray diffraction analysis was used to determine whether thesingle phase of a perovskite structure is decomposed or not, byannealing the samples for 61 hours in reduction atmospheres of 900° C.and 1000° C. In the sample No. 1 (x=0), the single phase of a perovskitestructure was decomposed into La₂O₃ (or La(OH)₃) and metal Fe. In thesample No. 2 (x=0.05), the single phase of a perovskite structure wasstable in a reduction atmosphere of 900° C., but the single phase of aperovskite structure was decomposed in a reduction atmosphere of 1000°C. to produce La₂O₃ and metal Fe. In the sample Nos. 3 to 9 (x≤0.1), thesingle phases of a perovskite structure were stable in reductionatmospheres of 900° C. and 1000° C., and there was no production ofLa₂O₃ and metal Fe. The sample in which the decomposition of the singlephase of a perovskite structure did not take place in the reductionatmosphere of 900° C. or 1000° C. was denoted by “good”, and the samplein which the decomposition of the single phase of a perovskite structuretook place in the reduction atmosphere of 900° C. or 1000° C. wasdenoted by “poor” and the results are shown in Table 1.

(5) Reactivity with Zirconia

Zirconia (ZrO₂) stabilized with Y₂O₃ added in an amount of 3 mol %(yttria-stabilized zirconia: YSZ) and raw materials of the samples weremixed, and then the resulting mixtures were sintered at 1300° C. Theproduced phase was identified by performing a powder X-ray diffractionanalysis (XRD, CuKα rays). In all of the samples, a reaction withzirconia did not occur. In Table 1, “good” shows that the reaction withzirconia did not occur. FIG. 1 shows one example of the peak intensityof each phase of sample No. 3 (x=0.1), obtained by XRD. In FIG. 1, Pindicates a peak of a perovskite phase and Z indicates a peak of azirconia phase.

(6) Ionic Conductivity

Green sheets of the samples of sample Nos. 2 to 8, each having acomposition in which x satisfies 0.05≤x≤0.5, were processed into adisc-shape having a diameter of 30 mm and a thickness of 10 mm. Afterthese green sheets were sintered, platinum electrodes were formed onboth sides of the sintered body. A humidified hydrogen gas (containingabout 4% of water vapor) of 30° C., which has an oxygen partial pressureequivalent to an oxygen partial pressure of a fuel gas, was blown to onesurface of the sintered body being maintained at 900° C. and air wasblown to the other surface of the sintered body to measure theelectromotive force of the sintered body as a hydrogen concentrationcell. While a voltage of up to 1.04 V (a theoretical value) is generatedin the case where oxygen-ionic conduction occurs, electromotive forcesof about 2 mV were generated in all of the evaluated samples.Consequently, it is estimated that 0.2% of all the electric conductivitycorresponds to ionic conduction. This level of ionic conduction is verysmall compared with electron conduction and can be neglected. In Table1, the fact that there is little ionic conductivity is represented by“good”.

TABLE 1 Electric Electric Electric Electric conductivity conductivityconductivity conductivity [Scm⁻¹] in [Scm⁻¹] in [Scm⁻¹] in [Scm⁻¹] inReduction Reduction oxidation reduction oxidation reduction SinteringSintering Reaction Sample stability stability atmospheres atmospheresatmospheres atmospheres property property with Ionic No. x (900° C.)(1000° C.) (900° C.) (900° C.) (1000° C.) (1000° C.) (1300° C.) (1400°C.) zirconia conductivity 1 0 poor poor 0.50 (very — 0.58 (very — goodgood good good good) good) 2 0.05 good poor 0.40 (very 0.70 (very 0.45(very — good good good good good) good) good) 3 0.1 good good 0.29 (very0.44 (very 0.33 (very 0.90 (very good good good good good) good) good)good) 4 0.15 good good 0.22 (very 0.37 (very 0.25 (very 0.71 (very goodgood good good good) good) good) good) 5 0.2 good good 0.16 (very 0.29(very 0.18 (very 0.51 (very good good good good good) good) good) good)6 0.3 good good 0.06 (very 0.12 (very 0.07 (very 0.24 (very poor goodgood good good) good) good) good) 7 0.4 good good 0.02 (poor) 0.06 (very0.05 (very 0.10 (very poor good good good good) good) good) 8 0.5 goodgood 0.01 (poor) 0.02 (poor) 0.02 0.03 poor poor good good (poor) (good)9 1 good good 0.0001 0.0001 0.0001 0.0001 poor poor good good (poor)(poor) (poor) (poor)

As shown in Table 1, sample Nos. 2 to 7 having the composition in whichx satisfies 0.05≤x≤0.4 are cases where x satisfies 0<x<0.5 in theceramic composition represented by the composition formulaLa(Fe_(1-x)Al_(x))O₃, and it was found based on the above-mentionedevaluations of bulk samples that these samples were chemically stable inboth oxidation atmospheres and reduction atmospheres, had no ionicconductivity and a high electron conductivity (electric conductivity),and can reduce the sintering temperature to about 1300 to 1400° C.Furthermore, it was found based on the above-mentioned evaluations ofbulk samples that sample Nos. 3 to 6 can provide a material which ischemically stable even in high-temperature reduction atmospheres of1000° C., and enables a higher electron conductivity (electricconductivity).

Next, bulk samples of the ceramic compositions represented by thecomposition formula Sr_(1-x)A_(x)Ti_(1-y)B_(y)O₃ in which A representsLa or Y, B represents Nb or Ta, and x and y each represent a molarratio, satisfying 0≤x≤0.5 and 0≤y≤0.5 were prepared at variouscomposition ratios as intermediate layer materials according to thefollowing procedure, and each sample was evaluated.

(Preparation of Bulk Sample)

In order to obtain a desired composition by setting the values of x andy, which are molar amount in the composition formulaA_(1-x)B_(x)Ti_(1-y)C_(y)O₃ in which A represents Sr, B represents La orY, C represents Nb or Ta, and x and y each represent a molar ratio,satisfying 0≤x≤0.5 and 0≤y≤0.5, at predetermined values within theabove-mentioned range, as starting materials of the sample, lanthanumoxide (La₂O₃), yttrium oxide (Y₂O₃), strontium carbonate (SrCO₃),titanium oxide (TiO₂), niobium oxide (Nb₂O₅), and tantalum oxide (Ta₂O₅)were weighed stoichiometrically and mixed. Water was added to themixture, and the resulting mixture was milled with zirconia balls andmixed. Thereafter, the resulting mixed powder was dried and calcined at1100° C. In order to form pores in the sample, carbon was added in anamount of 0 to 20% by weight to the obtained calcined powder, and anorganic solvent and a butyral-based binder were added and the resultingmixture was mixed to prepare a slurry. The slurry was formed into asheet by the doctor blade method. The resulting green sheet wassubjected to a binder removal treatment, and then was sintered by beingmaintained at 1300° C. to obtain a sample. The following evaluationswere performed with the obtained sample.

(Evaluation of Bulk Sample of Material for Intermediate Layer)

(1) X-Ray Diffraction

A produced phase was identified by performing a powder X-ray diffractionanalysis (XRD, CuKα rays) after calcination and after sintering of thesample. It was identified that all of the phases produced aftersintering were a single phase of a perovskite structure.

(2) Electrical Resistance

Slurries were prepared by mixing a polybutyral-based binder and amixture of ethanol and toluene as an organic solvent with each of thematerial powders of the following members. The slurries were formed intogreen sheets of the interconnector, the intermediate layer, the fuelelectrode layer and the air electrode layer by the doctor blade method.

Interconnector: A calcined powder of a starting material of(LaFe_(0.8)Al_(0.2)O₃) (x=0.2).

Intermediate layer: A_(1-x)B_(x)Ti_(1-y)C_(y)O₃ in which A representsSr, and x=0 and y=0, that is, a calcined powder of a starting materialof SrTiO₃.

Fuel electrode layer: A compound formed by adding 30 parts by weight ofa carbon powder to 100 parts by weight of a material powder made of amixture of 65% by weight of nickel oxide (NiO) and 35% by weight ofzirconia (ZrO₂) stabilized with yttria (Y₂O₃) added in an amount of 8mol % (yttria-stabilized zirconia: 8YSZ).

Air electrode layer: A compound formed by adding 30 parts by weight of acarbon powder to 100 parts by weight of a material powder composed ofLa_(0.8)Sr_(0.2)MnO₃.

The obtained green sheets of the interconnector and the intermediatelayer were processed into disc-shapes of 30 mm in diameter. The obtainedgreen sheets of the fuel electrode layer and the air electrode layerwere processed into disc-shapes of 15 mm in diameter. The green sheetsof the interconnector, the intermediate layer and the fuel electrodelayer were stacked sequentially on the disc-shaped green sheet of theair electrode layer. The thicknesses of the respective green sheets wereset in such a way that the thicknesses of the sintered green sheets were50 μm for the air electrode layer, 300 μm for the interconnector, 50 μmfor the fuel electrode layer, and three kinds of 9 μm, 30 μm and 50 μmfor the intermediate layer.

The stacked layers were press-bonded to one another at a pressure of1000 kgf/cm² by undergoing warm isostatic pressing (WIP) at atemperature of 80° C. for 2 minutes. The press-bonded body was subjectedto a binder removal treatment within the range of 400 to 500° C., andthen was sintered by being maintained at 1300° C. for 3 hours.

A probe provided with a platinum net was pressed against the surfaces ofthe air electrode layer and the fuel electrode layer of the obtainedsamples of sintered laminates (sample Nos. 11 to 13), and electricalresistances of the samples were measured at 900° C. while passing air onan air electrode layer side and a humidified hydrogen gas (containingabout 4% of water vapor) of 30° C. on a fuel electrode layer side. Sincecarbon was not added to the calcined powder as the material powder ofthe intermediate layer, the open porosity of the intermediate layer was0%.

For comparison, a sample of sintered laminate (sample No. 10) wasprepared by sintering a stacked layers formed by stacking aninterconnector and a sheet of a fuel electrode layer sequentially on adisc-shaped green sheet of an air electrode layer, in the same manner asin the above-mentioned samples, and its electrical resistances wasmeasured.

The measurement results of the electrical resistance are shown in Table2.

TABLE 2 Sample No. 10 11 12 13 Thickness of 0 9 30 50 intermediate layer[μm] Electrical 168.9 3.0 3.2 3.4 resistance [Ωcm]

It is evident from Table 2 that while the comparative example (sampleNo. 10), formed by stacking the air electrode layer, the interconnectorand the fuel electrode layer sequentially, exhibited a high electricalresistance, the samples as examples of the present invention (sampleNos. 11 to 13), formed by stacking the air electrode layer, theinterconnector, the intermediate layer and the fuel electrode layersequentially, exhibited a considerably low electrical resistance andthat the smaller the thickness of the intermediate layer, the lower theelectrical resistance.

(3) Effect of Pores

In order to form pores having different porosities in the samples of theintermediate layer, carbon was added within the range of 0 to 20% byweight to a calcined powder of a starting material of SrTiO₃ as amaterial powder of the intermediate layer, and an organic solvent and abutyral-based binder were added and the resulting mixture was mixed toprepare a slurry. Then, samples of sintered laminates (sample Nos. 14 to17) were prepared in the same manner as in the above-mentioned samplesand their electrical resistances were measured. The thickness of thesintered intermediate layer was adjusted to 50 μm. The porosity wasmeasured according to an Archimedes method.

The measurement results of the electrical resistance are shown in Table3.

TABLE 3 Sample No. 14 15 16 17 Open 10 20 30 40 porosity [%] Electrical3.6 3.7 3.9 4.1 resistance [Ωcm]

It is evident from Table 3 that when the porosity is made high, theelectrical resistance of the sample of sintered laminate slightlyincreases, but there is no problem with the level of the electricalresistance.

(4) Effect of Dopants

In order to prepare a sample of the intermediate layer composed of amaterial obtained by replacing a part of Sr and Ti in SrTiO₃, an organicsolvent and a butyral-based binder were added to a calcined powder of astarting material of Sr_(1-x)B_(x)Ti_(1-y)C_(y)O₃ (B represents La or Y,C represents Nb or Ta, x=0.2 and y=0.2) as a material powder of theintermediate layer, and the resulting mixture was mixed to prepare aslurry. Then, samples of sintered laminates (sample Nos. 18 to 21) wereprepared in the same manner as in the above-mentioned samples and theirelectrical resistances were measured. Since carbon was not added to thecalcined powder as the material powder of the intermediate layer, theopen porosity of the intermediate layer was 0%. Further, the thicknessof the sintered intermediate layer was adjusted to 50 μm.

The measurement results of the electrical resistance are shown in Table4.

TABLE 4 Sample No. 18 19 20 21 B La Y — — C — — Nb Ta Electrical 2.9 2.92.9 2.9 resistance [Ωcm]

It is evident from Table 4 that the electrical resistance of the sampleof sintered laminate is reduced in the sample of the intermediate layercomposed of a material obtained by replacing a part of Sr and Ti inSrTiO₃.

(Preparation and Power Generation Test of Fuel Cell Sample)

Next, a sample of a planar solid electrolyte fuel cell was prepared byusing a ceramic composition having the composition (x=0.2) shown in thesample No. 5 in Table 1 as an interconnector material and a powergeneration test of the sample was performed.

First, material powders of members constituting the samples of solidelectrolyte fuel cells 1 shown in FIGS. 6 to 8 were prepared in thefollowing manner.

Fuel electrode layer 11: A compound formed by adding 30 parts by weightof a carbon powder to 100 parts by weight of a material powder made of amixture of 65% by weight of nickel oxide (NiO) and 35% by weight ofzirconia (ZrO₂) stabilized with yttria (Y₂O₃) added in an amount of 8mol % (yttria-stabilized zirconia: 8YSZ).

Solid electrolyte layer 12: Zirconia (ZrO₂) stabilized with yttria(Y₂O₃) added in an amount of 10 mol % (yttria-stabilized zirconia:10YSZ).

Air electrode layer 13: A compound formed by adding 30 parts by weightof a carbon powder to 100 parts by weight of a material powder composedof La_(0.8)Sr_(0.2)MnO₃.

Support structure 14: Y_(0.15)Ta_(0.15)Zr_(0.7)O₂ (zirconia (ZrO₂)stabilized with Y₂O₃ added in an amount of 7.5 mol % and Ta₂O₅ added inan amount of 7.5 mol %) (electrically insulating material).

Interconnector 15: A calcined powder of a starting material of(LaFe_(0.8)Al_(0.2)O₃) (x=0.2).

Intermediate layer 18: A compound formed by adding a predeterminedamount of a carbon powder to a material powder made of a calcined powderof a starting material of SrTiO₃ so as to give a porosity of 20%.

First, the thermal expansion of the bulk samples of the interconnector15, the solid electrolyte layer 12 and the support structure 14 wasinvestigated by a thermomechanical analyzer (TMA). The measurementresults are shown in FIG. 2, which is a graph showing the relationshipsbetween temperature (° C.) and thermal expansion (ΔL/L) (%). In thegraph of FIG. 2, lines indicated by “x=0.1”, “Electrolyte” and “SupportStructure” show the relationships between temperature and thermalexpansion in the bulk samples of the interconnector 15, the solidelectrolyte layer 12 and the support structure 14. For example, the bulksamples of the interconnector 15, the solid electrolyte layer 12 and thesupport structure 14 had thermal expansion coefficients of 11.1×10⁻⁶/K,10.2×10⁻⁶/K, and 10.5×10⁻⁶/K, respectively, at 1000° C. in air. It isfound that difference in the thermal expansion between the bulk samplesof the interconnector 15 and the solid electrolyte layer 12 is small. Itis found that difference in the thermal expansion between the bulksamples of the interconnector 15 and the support structure 14 isparticularly small.

Next, material powders of the interconnector 15 and the solidelectrolyte layer 12 were used and subjected to co-sintering at 1300° C.The planar dimension of the sintered body was 63 mm×49 mm, the thicknessof the solid electrolyte layer 12 was 200 μm, and the thickness of theinterconnector 15 was 200 μm. In the obtained sintered body, peelingbetween the solid electrolyte layer 12 and the interconnector 15 was notfound, and the solid electrolyte layer 12 was firmly joined to theinterconnector 15. The sintered body was heated to 1000° C.,heating/cooling at a rate of 10° C./min were repeated, and then thesintered body was kept at 1000° C. for 24 hours, but peeling did nottake place between the solid electrolyte layer 12 and the interconnector15. Therefore, it is evident that the difference in the thermalexpansion between the bulk samples of the interconnector 15 and thesolid electrolyte layer 12, respectively measured as described above,presents no problem.

As shown in FIG. 6, by use of the materials thus prepared, green sheetsof the fuel electrode layer 11, the solid electrolyte layer 12, the airelectrode layer 13, the support structure 14, the interconnector 15, andthe intermediate layer 18 were prepared in the following manner.

The material powders, a polyvinyl butyral-based binder and a mixture(weight ratio is 1:4) of ethanol and toluene as an organic solvent weremixed, and then the resulting mixtures were formed into green sheets bya doctor blade method.

As shown by dashed lines in FIG. 6, elongated through-holes 21 a and 22a respectively for forming a fuel gas supply channel 21 and an airsupply channel 22 shown in FIG. 7 were formed in the green sheet of thesupport structure 14 by puncturing the green sheet with a mechanicalpuncher.

Further, fitting portions 11 a, 12 a and 13 a into which the greensheets of the fuel electrode layer 11, the solid electrolyte layer 12and the air electrode layer 13 were to be fitted, respectively, wereformed in the green sheet of the support structure 14 on which the fuelelectrode layer 11, the solid electrolyte layer 12 and the air electrodelayer 13 were to be arranged.

Moreover, fitting Portions 31 a and 32 a into which the green sheets ofthe current collecting layer 31 of fuel electrode and the currentcollecting layer 32 of air electrode were to be fitted, respectively,were formed in the green sheet of the support structure 14 on which thecurrent collecting layer 31 of fuel electrode and the current collectinglayer 32 of air electrode were to be arranged. The green sheet of thecurrent collecting layer 31 of fuel electrode was prepared by using amaterial having the same composition as that of the material powder ofthe fuel electrode layer 11 and the green sheet of the currentcollecting layer 32 of air electrode was prepared by using a materialhaving the same composition as that of the material powder of the airelectrode layer 13.

Furthermore, as shown by dashed lines in FIG. 6, elongated through-holes21 a and 22 a respectively for forming a fuel gas supply channel 21 andan air supply channel 22 shown in FIG. 7 were formed in the green sheetsof the interconnector 15 and the intermediate layer 18 by puncturing thegreen sheets with a mechanical puncher.

In the green sheets of the support structures 14 thus prepared, thegreen sheet of the interconnector 15 was fitted into the through-hole 15a, the green sheets of the fuel electrode layer 11, the solidelectrolyte layer 12 and the air electrode layer 13 were fitted into thefitting portions 11 a, 12 a and 13 a, respectively, and the green sheetsof the current collecting layer 31 of fuel electrode and the currentcollecting layer 32 of air electrode were fitted into the fittingportions 31 a and 32 a, respectively. The green sheets of theinterconnector 15 and the intermediate layer 18 were stackedsequentially on three green sheets thus obtained as shown in FIG. 7. Inaddition, the thicknesses of the respective green sheets were set insuch a way that the thicknesses of the sintered green sheets were 50 μmfor the fuel electrode layer 11, 50 μm for the solid electrolyte layer12, 50 μm for the air electrode layer 13, 300 μm for the interconnector15, 50 μm for the intermediate layer 18, 250 μm for the currentcollecting layer 31 of fuel electrode, and 250 μm for the currentcollecting layer 32 of air electrode.

The stacked layers were press-bonded to one another at a pressure of1000 kgf/cm² warm isostatic pressing (WIP) at a temperature of 80° C.for 2 minutes. The press-bonded body was subjected to binder removaltreatment within the range of 400 to 500° C., and then was sintered bybeing maintained at 1300° C. for 3 hours.

The resulting sample of the solid electrolyte fuel cell 1 was heated to900° C., and a humidified hydrogen gas (containing about 4% of watervapor) of 30° C. and air were supplied through a fuel gas supply channel21 and an air supply channel 22, respectively, to perform a powergeneration test and an open circuit voltage (OCV) was measured. The opencircuit voltage was 1.07 V, which was equal to a theoretical value, andcell impedance was small. Both the open circuit voltage and theimpedance did not change even after applying a current at a currentdensity of 0.4 A/cm². It is evident from this result that the solidelectrolyte fuel cell 1 including the interconnector 15 can form a densebody without producing cracks by co-sintering, and particularly, did notform a high-resistive layer not only between the interconnector 15 andthe air electrode layer 13, but also between the interconnector 15 andthe fuel electrode layer 11 and provided good electrical connection.

Embodiments and examples disclosed herein are to be construed to beillustrative in all respects and but not restrictive. The scope of thepresent invention is defined by the appended claims rather than by thepreceding embodiments and examples, and all modifications and variationsequivalent to bounds of the claims and within bounds of the claims areintended to be embraced by the present invention.

INDUSTRIAL APPLICABILITY

Since it is possible to obtain an interconnector material which ischemically stable in both oxidation atmospheres and reductionatmospheres, has a low ionic conductivity and a high electronconductivity (electric conductivity), and can be densified at a lowtemperature of about 1300 to 1400° C., by using the interconnectormaterial, it is possible to obtain an intercellular separationstructure, which is chemically stable in a high-temperature environmentof 800 to 1000° C., the operating temperature of a solid electrolytefuel cell, and in both oxidation atmospheres and reduction atmospheres,and a solid electrolyte fuel cell including the intercellular separationstructure.

DESCRIPTION OF REFERENCE SYMBOLS

1: solid electrolyte fuel cell, 11: fuel electrode layer, 12: solidelectrolyte layer, 13: air electrode layer, 14: support structure, 15:interconnector, 18: intermediate layer, 21: fuel gas supply charnel, and22: air supply channel.

The invention claimed is:
 1. A cell structure comprising: a plurality ofcells which each comprise an anode layer, a solid electrolyte layer, acathode layer stacked sequentially; and an intercellular separationstructure disposed between cells of the plurality of cells, theintercellular separation structure comprising an electrical insulatordisposed to separate an anode gas and a cathode gas which are suppliedto each of the plurality of cells and an electrical conductor in theelectrical insulator electrically connecting the plurality of cells toeach other, and the electrical conductor comprising a first conductivebody electrically connected to the anode layer, a second conductive bodyelectrically connected to the cathode layer, and an interconnector ofdense material through which a gas does not a pass disposed to connectthe first conductive body and second conductive body.
 2. The solidelectrolytic fuel cell structure according to claim 1 wherein the anodelayer comprises a fuel electrode and the cathode layer comprises an airelectrode, the first conductive body contacts the anode layer throughthe fuel electrode, and the second conductive body contacts the cathodelayer through the air electrode.
 3. The solid electrolyte fuel cellaccording to claim 2, wherein the anode layer comprises nickel.
 4. Thesolid electrolytic fuel cell structure according to claim 3 wherein theinterconnector comprises a ceramic component electrical conductor havinga compositional formula of La(Fe_(1-x)Al_(x))O₃ in which 0<x<0.5.
 5. Thesolid electrolytic fuel cell structure according to claim 2 wherein theinterconnector comprises a ceramic component electrical conductor havinga compositional formula of La(Fe_(1-x)Al_(x))O₃ in which 0<x<0.5.
 6. Thesolid electrolytic fuel cell structure according to claim 5, wherein0.1≤x≤0.3.
 7. The solid electrolytic fuel cell structure according toclaim 1 wherein the interconnector comprises a ceramic componentelectrical conductor having a compositional formula ofLa(Fe_(1-x)Al_(x))O₃ in which 0<x<0.5.
 8. The solid electrolytic fuelcell structure according to claim 5, wherein 0.1≤x≤0.3.
 9. The solidelectrolyte fuel cell according to claim 8, wherein the anode layercomprises nickel.
 10. The solid electrolyte fuel cell according to claim1, wherein the anode layer comprises nickel.
 11. The solid electrolytefuel cell according to claim 1, further comprising: a first currentcollecting layer disposed between the anode layer and the firstconductive body; and a second current collecting layer disposed betweenthe cathode layer and the second conductive body.